In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for forming an integrated circuit comprising such a fine pattern. However, the conventional photolithography method has been close to the resolution limit, while microsizing of semiconductor devices has been accelerated. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography (EUVL), which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, is considered to be promising as an exposure technique for 45 nm or below. In this specification, “EUV light” means a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of from about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
EUV light is apt to be absorbed by any substances and the refractive indices of substances are close to 1 at this wavelength, whereby it is impossible to use a dioptric system like a conventional photolithography employing visible light or ultraviolet light. For this reason, for EUV light lithography, a catoptric system, i.e. a combination of a reflective photomask and a mirror, is employed.
A mask blank is a stacked member for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, formed thereon in this order. As the reflective layer, a multilayer reflective film is usually employed wherein a high refractive index layer and a low refractive index layer are alternately laminated to enhance the light reflectance when the layer surface is irradiated with EUV light. For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing Ta or Cr as the main component, is employed.
On the absorber layer of an EUV mask blank, a low reflective layer to a mask pattern inspection light is usually provided. A mask pattern after formed is inspected for pattern defects by using light in a wavelength region (190 to 260 nm) of a deep ultraviolet light. In the pattern inspection employing the light in the above wavelength region, pattern defects are detected by the difference in the reflectance between a portion where the low reflective layer and the absorber layer were removed by the patterning step and a region where the low reflective layer and the absorber layer remain, that is, the contrast of reflected light on the surface of these portions. In order to improve the sensitivity for inspection of a mask pattern, it is required to increase the contrast, and for that purpose, the low reflective layer is required to have low reflection properties in the above wavelength region, that is, it is required to have a reflectance in the above wavelength region of at most 15%.
Patent Document 1 discloses that formation of a low reflective layer comprising an oxide of a tantalum/boron alloy (TaBO) or an oxynitride of a tantalum/boron alloy (TaBNO) on an absorber layer comprising a nitride of a tantalum/boron alloy (TaBN) is preferred in view of a low reflectance in a wavelength region (190 nm to 260 nm) of the inspection light for a mask pattern.
Further, Patent Documents 2 and 3 disclose that formation of a low reflective layer comprising a metal, silicon (Si), oxygen (O) and nitrogen (N) on an absorber layer is preferred so as to adjust the reflectance in a wavelength region (190 nm to 260 nm) of the inspection light for a mask pattern.
Patent Document 1: JP-A-2004-6798
Patent Document 2: JP-A-2006-228767
Patent Document 3: JP-A-2007-335908